1. Field of the Invention
The present invention relates generally to a wireless communication system, and more particularly, to a device and method for transmitting uplink sounding reference signals for a wireless communication system.
2. Description of the Related Art
An objective of the Third Generation Partnership Project (3GPP) standardization organization is to establish a new generation of communication standard, known as the Long Term Evolution (LTE) standard. The downlink transmission technique of LTE is based on Orthogonal Frequency Division Multiplexing (OFDM), while the uplink transmission technique is based on a Single Carrier Frequency Division Multiple Access (SCFDMA) scheme. There are two types of frame structures in the LTE system, wherein type 1 applies Frequency Division Duplex (FDD) and type 2 applies Time Division Duplex (TDD).
FIG. 2 illustrates a frame structure in the LTE FDD system where time duration of a radio frame is 307200×Ts=10 ms and each frame is divided into 20 time slots 15360 Ts=0.5 ms long which cover the index ranging from 0 to 19. Each time slot includes several OFDM symbols and use a Cyclic Prefix (CP) of one of two types, i.e., normal CP and extended CP. Time slots using normal CP include seven OFDM symbols while the time slots using extended CP have six OFDM symbols. Each sub-frame consists of two continuous time slots, i.e., the kth sub-frame includes the 2kth and (2k+1)th time slots.
FIG. 3 illustrates a frame structure in the LTE TDD system. A radio frame having a length of 307200×Ts=10 ms is divided into two equal half-frames 153600×Ts=5 ms long. Each half-frame includes eight 15360Ts=0.5 ms long slots and three special domains, i.e., a Downlink Pilot Time Slot (DwPTS), a Guard Period (GP) and an Uplink Pilot Time Slot (UpPTS), and has a total length of 30720Ts=1 ms. Each time slot includes several OFDM symbols and use either the normal CP or the extended CP. Time slots using normal CP include 7 OFDM symbols while the time slots using extended CP have 6 OFDM symbols.
Each sub-frame consists of two continuous time slots, i.e., the kth sub-frame includes the 2kth and (2k+1)th time slots. Sub-frame 1 and 6 include the aforementioned three domains. To this point, sub-frames 0, 5 and DwPTS are constantly assigned for downlink transmission. If the conversion period is 5 ms, UpPTS, sub-frames two and seven are constantly assigned for uplink transmission. If the conversion period is 10 ms, UpPTS and sub-frame 2 are constantly assigned for uplink transmission.
FIG. 4 illustrates a configuration diagram of an LTE TDD frame structure. In FIG. 4, it can be clearly seen that in configuration 0, each radio frame contains ten radio sub-frames that are circularly indexed from 0. Both sub-frames 0 and 5 are adopted to transmit downlink data, i.e., both sub-frames 0 and 5 are adopted by evolved Node B (eNB) to transmit information to UEs, Sub-frames 2, 3, 7, 8 and 9 are adopted by UEs to transmit uplink data, i.e., to transmit information to the eNB, and Sub-frames 1 and 6, also known as special sub-frames, are composed of three special time slots defined as DwPTS, GP and UpPTS respectively. Here, the time length of DwPTS, GP and UpPTS is variable depending on the system configuration.
FIG. 5 illustrates a distribution diagram of the time-frequency grid of a single uplink sub-frame and a possible location of the time-frequency resource for the Sounding Reference Signal (SRS) transmission under the condition that the normal CP and the extended CP are configured in an LTE system. When the system is configured with normal CPs, each uplink sub-frame within a Resource Block (RB) contains two time slots with each containing 7 Single Carrier Frequency Division Multiple Access (SC-FDMA) symbols (the time domain) and 12 sub-carriers (the frequency domain). When the system is configured with extended CPs, each uplink sub-frame within each RB contains two continuous time slots containing 6 SC-FDMA symbols and 12 sub-carriers. The minimum uplink sub-frame resource is called the Resource Element (RE).
According to present discussion on LTE, in each radio frame, the last symbol in some sub-frame is adopted to transmit the SRS.
An objective of LTE on an uplink SRS is for the SRS frequency-hop scheme to guarantee that the SRS signal from the UE for the entire system bandwidth is as loudly as possible. At present, with the provision of SRS for four different system bandwidth configurations, eNB adopts 8-bit Radio Resource Control (RRC) signaling to assign the UE with different frequency-hop schemes. Among the 8-bit RRC signaling, four bits are adopted to indicate the configuration of SRS bandwidth, two bits are adopted to indicate the UE's SRS bandwidth in current configuration, and the remaining two bits are adopted to indicate SRS frequency-hop bandwidth.
To avoid collision among SRSs of different UEs within the same frequency-hop period (T), the same transmission sub-frame offset and the same Comb location, when a UE configures the SRS frequency hop, the SRS's logic IDentifier (ID) (nSRS) is calculated according to a current radio frame number (nf), an index (ns) of the time slot for transmitting the SRS and the SRS period (T), by Equation (1) as follows:nSRS=└(nf×10+└ns/2┘)/T┘  (1)
The physical resource for each transmission of SRS is then determined based on nSRS. When nSRS is a continuous value, according to the present SRS frequency-hop scheme, the UE can be guaranteed to sound the entire system bandwidth as loudly as possible. In an FDD system, since frequency division multiplex is applied in both uplink and downlink, it can be guaranteed that in each SRS period at least one uplink sub-frame is allocated by eNB. Therefore, nSRS obtained by Equation (1) is a continuous value. This guarantees that the SRS could sound the entire system bandwidth.
However, in a TDD system's frame structure, it cannot be guaranteed that at least one uplink sub-frame is allocated in every two continuous sub-frames. Thus, when a UE is configured with a 2 ms period, nSRS obtained by Equation (1) is not continuous in a TDD system, which results in that the UE is not able to sound the entire system bandwidth or that the frequency-hop pattern is not uniform within the sounding frequency-hop bandwidth.
FIGS. 9A and 9B illustrate the values of nSRS obtained by Equation (1) when T=2 (in FIG. 9A) and 5 respectively. FIG. 10 illustrates the problems in the current system when the system bandwidth=25RB, the SRS frequency-hop index=3, the SRS bandwidth=4 and the SRS frequency-hop bandwidth=20. Since seven different uplink and downlink configurations can be applied in a TDD system, the new frequency-hop scheme should guarantee that within any SRS configuration period in either TDD or FDD system, the UE could periodically sound the entire SRS frequency-hop bandwidth with a fixed period, and the frequency-hop pattern should be uniformly distributed over the frequency-hop bandwidth, i.e., the sounding frequency for each SRS bandwidth should be as consistent as possible. As indicated in FIGS. 9A and 9B, the SRS only sounds a part of the bandwidth or the numbers of sounding for the bandwidths are not equal, regardless of the value of T.
At present, there is no LTE discussion underway on how to solve the complicated problem of all-around consideration on the seven different uplink and downlink configurations.